Trace metal extractability and bioaccessibility in urban soils

Abstract Purpose One of the most challenging issues in urban soils is the accumulation of pollutants such as heavy metals, which could reach the natural waters or enter the food chain through plant uptake. In order to assess the health risk related it is necessary to know their availability. Methods We analyzed the chemical extractability of five trace metals (Pb, Cu, Zn, Ni and Cr) in 55 soils from Santiago de Compostela (Spain) with diverse land use (urban grassland, urban forest, urban agriculture) and parent material lithology (granite, schist, gneiss, amphibolite). Soluble metals were evaluated using an extraction with 0.01 M CaCl 2 , plant-available metals were obtained after extraction with EDTA, and bioaccessibility was assessed following the USEPA in vitro extraction with glycine. Results Metal extractability was in general higher in the USEPA method than in EDTA, and much higher than in CaCl 2 . Among the elements studied, only Zn was detected consistently in CaCl 2 extract, with values always lower than 3% of the total contents, in a decreasing sequence Zn > Pb > Ni > Cu > Cr. Concentrations of plant-available metals followed a decreasing sequence Pb > Cu > Zn > Ni > Cr, with values that represented, on average, between 1 and 23% of their total concentrations. Bioaccessibility followed a similar sequence: Pb > Cu > Zn > Ni > Cr, with values that ranged between 2 and 55% of the total concentrations. Plant-available and bioaccessible Cu and Zn were higher in urban garden soils with respect to other uses. Conclusion The availability of trace metals in these soils is very low and supports previous hypotheses about their sources, with Cu, Pb and Zn coming from anthropogenic pollution and Ni and Cr from natural sources related to the soils parent material.